This invention relates to metal oxide varistors and, more particularly, to a method of controlling the size of the metal oxide grains in varistors and thus to provide more uniform and improved devices.
In general, the current flowing between two spaced points is directly proportional to the potential difference between those points. For most known substances, current conduction therethrough is equal to the applied potential difference divided by a constant, which has been defined by Ohm's law to be its resistance. There are, however, a few substances which exhibit non-linear resistance. Some devices, such as metal oxide varistors, utilize these substances and require resort to the following equation (1) to quantitatively relate current and voltage: ##EQU1## where V is the voltage applied to the device, I is the current flowing through the device, C is a constant and .alpha. is an exponent greater than 1. Inasmuch as the value of .alpha. determines the degree of non-linearity exhibited by the device, it is generally desired that .alpha. be relatively high. .alpha. is calculated according to the following equation (2): ##EQU2## where V.sub.1 and V.sub.2 are the device voltages at given currents I.sub.1 and I.sub.2, respectively.
At very low voltages and very high voltages metal oxide varistors deviate from the characteristics expressed by equation (1) and approach linear resistance characteristics. However, for a very broad useful voltage range the response of metal oxide varistors is as expressed by equation (1).
The values of C and .alpha. can be varied over wide ranges by changing the varistor formulation or the manufacturing process. Another useful varistor characteristic is the varistor voltage which can be defined as the voltage across the device when a given current is flowing through it. It is common to measure varistor voltage at a current of one milliampere and subsequent reference to varistor voltage shall be for voltage so measured. The foregoing is, of course, well known in the prior art.
Metal oxide varistors are usually manufactured as follows. A plurality of additives is mixed with a powdered metal oxide, commonly zinc oxide. Typically, four to twelve additives are employed, yet together they comprise only a small portion of the end product, for example less than five to ten mole percent. In some instances the additives comprise less than one mole percent. The types and amounts of additives employed vary with the properties sought in the varistor. Copious literature describes metal oxide varistors utilizing various additive combinations. For example, see U.S. Pat. No. 3,663,458. A portion of the metal oxide and additive mixture is then pressed into a body of a desired shape and size. The body is then sintered for an appropriate time at a suitable temperature as is well known in the prior art. Sintering causes the necessary reactions among the additives and the metal oxide and fuses the mixture into a coherent pellet. Leads are then attached and the device is encapsulated by conventional methods.
A problem encountered in the manufacture of metal oxide varistors by the prior art method is the inability to precisely predict and control the properties of the device. Thus, manufacturing yield is a matter of concern to varistor manufacturers. It is known that commercially available metal oxide varistors are granular in structure. A consideration of grain structure and grain size will furnish an example of the inability of manufacturers to control the final device properties. While the conduction process in metal oxide varistors is not fully understood, it is believed that the mechanism creating the varistor action takes place at the intergranular phase that separates the grains in the finished varistor. It was reasoned therefore that the varistor voltage is at least in part dependent upon the average number of intergranular regions between the two contacts. Thus, it was felt that controlling the number of intergranular regions would aid in controlling the varistor voltage. Efforts were made to embody this theory in varistors by controlling the grain size in the finished varistors and thus controlling the number of intergranular regions. However, it was found that existing manufacturing techniques were inadequate to control the grain size with sufficient accuracy to yield improved devices. For example, one method explored in an effort to make a low voltage varistor was controlling the sintering process so that the grain size became relatively large. Unfortunately, individual grains often became too large and established a current path between the contacts with very few intergranular regions. Upon conduction of the varistor, the bulk of the current passed through this preferred path with few interfaces creating an unacceptably high current density therein and leading to device failure. In summary, it has not heretofore been possible to exert a precise enough control over grain size to utilize the effect that grain size is believed to have on varistor voltage.
It is, therefore, an object of this invention to provide a varistor and a method for the fabrication thereof wherein the grain size in the varistor is simply and accurately controlled and thus to permit precise prediction of device properties.